Infrared detection technology plays very important roles in the modern national defense technology, such as missile guidance, infrared observation, targeting, and other areas. Infrared dome is an important component for the missile seeker, which protects the internal components of the system and transfers the infrared signal at the same time. ZnS is common material for both mid-wave and long-wave infrared domes because of its wide transmittance range. At present, chemical vapor deposition (CVD) and hot pressing sintering are the main methods for ZnS manufacture. The CVD method can yield products with good optical quality, but the growth period is long and the cost is high. The CVD method is suitable for the growth of flat products but have difficulty for domes with cavities.
Hot pressing sintering, another technique for producing ZnS, is short in production cycle. However, the method is suitable for flat sheet with uniform thickness. FIG. 1 shows the schematic section view of the existing technology for ZnS production by hot pressing sintering.
The hot pressing mold 20 consists of outer die 23, upper punch 21, and lower punch 22. The flat pressing ZnS product is restricted in the space surrounded by outer die 23, upper punch 21, and lower punch 22. During the sintering process, the hydraulic pressure is transmitted to the upper punch 21 and the lower punch 22, and hence compresses the products. However, for uneven samples, especially for the domes with cavities, the main issue for hot pressing sintering is the difference in compress ratio, resulted from the variation of thickness along the pressing direction. The thin part with large compress ratio is compressed strongly, while contrary for the thick part. So, such difference will leads to pressure variation, as well as micro-structure and property variation on products. Generally, the pressure of ZnS hot pressing is very high. For example, in “Hot pressing polycrystalline ZnS,” New Inorganic Materials, 1978, 45-47, ZnS with transmittance at 8-14 μm was prepared by hot pressing sintering under 300-400 MPa at 700-850° C. JP61-205659 (1986.9.11) discloses the hot pressing of infrared polycrystalline ZnS under 146-292 MPa at 770-965° C. Under such high pressure, the difference in compress ratio will often causes residual stress and cracks. Therefore, it is difficult to obtain infrared transparent ZnS domes with uniform micro-structures and properties. In the present technology, high temperature alloy materials are usually used as hot pressing mold, such as GB934421A (hot pressing polycrystalline zinc sulfide, the Journal of inorganic materials 1973-3-25), metal molybdenum, GH4049, K403, GH4037, pyromet 625, etc. The purity of ZnS powders for infrared materials is no less than 99.95% with metal ion impurity content <0.01%, according to the disclosed prior art.
JPH07242910A discloses a hot pressing sintering method using flowable particles to transfer pressure, in which the products are embodied in high melting point particles. However, in the method, the compress ratios are different in pressure direction and the vertical direction. Secondly, the method cannot be used in the manufacture of sintered bodies with cavities, such as the domes, because of different compress ratios. Furthermore, the products need to be pre-sintered without pressure in the method. However, the cubic ZnS transforms to hexagonal wurtzite phase when the temperature is elevated up to 1020° C. The ZnS powder cannot be pre-sintered evidently under the transform point without external pressure. Hence, the method cannot be used to manufacture infrared ZnS domes.